1. Field of the Invention
The present invention relates to a plasmon generator for use in thermally-assisted magnetic recording in which a recording medium is irradiated with near-field light to lower the coercivity of the recording medium for data writing, and to a thermally-assisted magnetic recording head including the plasmon generator.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head unit including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head unit including an induction-type electromagnetic transducer for writing are stacked on a substrate. In a magnetic disk drive, a thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
To increase the recording density of a magnetic recording device, it is effective to make the magnetic fine particles of the recording medium smaller. Making the magnetic fine particles smaller, however, causes the problem that the magnetic fine particles drop in the thermal stability of magnetization. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a recording medium having high coercivity. When writing data, a write magnetic field and heat are simultaneously applied to the area of the recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. The area where data is written subsequently falls in temperature and rises in coercivity to increase the thermal stability of magnetization. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light. The laser light to be used for generating near-field light is typically guided through a waveguide, which is provided in the slider, to the plasmon generator disposed near a medium facing surface of the slider.
The plasmon generator has a front end face located in the medium facing surface. The front end face generates near-field light. Surface plasmons are excited on the plasmon generator and propagate along the surface of the plasmon generator to reach the front end face. As a result, the surface plasmons concentrate at the front end face, and near-field light is generated from the front end face based on the surface plasmons.
U.S. Patent Application Publication No. 2011/0170381 A1 discloses a technology in which the surface of a waveguide and the surface of a metallic structure (plasmon generator) are arranged to face each other with a gap therebetween, and evanescent light that occurs at the surface of the waveguide based on the light propagating through the waveguide is used to excite surface plasmons on the metallic structure, so that near-field light is generated based on the excited surface plasmons. Further, U.S. Patent Application Publication No. 2011/0170381 A1 discloses forming a part of the metallic structure from a material different from that of other parts of the metallic structure.
Materials that are typically employed for plasmon generators are metals having high electrical conductivities, such as Au and Ag. However, Au and Ag are relatively soft and have relatively high thermal expansion coefficients. Thus, the following problems arise if a plasmon generator is formed entirely of Au or Ag.
In the process of manufacturing a thermally-assisted magnetic recording head, the medium facing surface is formed by polishing. During polishing, polishing residues of metal materials may grow to cause smears. To remove the smears, the polished surface is slightly etched by, for example, ion beam etching in some cases. If the plasmon generator is formed entirely of Au or Ag, which are relatively soft, the polishing and etching mentioned above may cause the front end face of the plasmon generator to be significantly recessed relative to the other parts of the medium facing surface. In such a case, the front end face of the plasmon generator becomes distant from the recording medium, and the heating performance of the plasmon generator is thus degraded.
Part of the energy of light guided to the plasmon generator through the waveguide is transformed into heat in the plasmon generator. Part of the energy of near-field light generated by the plasmon generator is also transformed into heat in the plasmon generator. The plasmon generator thus rises in temperature during the operation of the thermally-assisted magnetic recording head. If the plasmon generator is formed entirely of Au or Ag, the rise in temperature of the plasmon generator causes the plasmon generator to expand and significantly protrude toward the recording medium. This in turn may cause a protective film covering the medium facing surface to come into contact with the recording medium and thereby damage the recording medium or be broken. When the protective film is broken, the plasmon generator may be damaged by contact with the recording medium or may be corroded by contact with high temperature air.
Further, the plasmon generator formed entirely of Au or Ag may be deformed due to aggregation when its temperature rises. In addition, such a plasmon generator expands when its temperature rises and then contracts when its temperature drops. When the plasmon generator undergoes such a process, the front end face of the plasmon generator may be significantly recessed relative to the other parts of the medium facing surface. In such a case, the heating performance of the plasmon generator is degraded as mentioned above.
For the various reasons described above, the plasmon generator formed entirely of Au or Ag has the drawback of being low in reliability.
U.S. Patent Application Publication No. 2011/0170381 A1 discloses a metallic structure composed of a main body and a layer having a greater hardness than the main body (this layer will hereinafter be referred to as the hard layer). In this metallic structure, the main body is not exposed in the medium facing surface, but the hard layer is exposed in the medium facing surface. In this metallic structure, surface plasmons are generated in the main body. The generated surface plasmons propagate to the hard layer, and near-field light is generated from the vertex of the hard layer. This metallic structure has the drawback that there is a great loss of the surface plasmons as they propagate from the main body to the hard layer, and it is thus difficult to allow the surface plasmons to efficiently propagate to the vertex.